Video Transcript
Which subtraction sentence matches
the calculation on this number line? 80 take away 35 equals 27, 80 take
away 53 equals 27, or 80 take away 63 equals 27.
If we’re ever asked to subtract two
numbers, we can often find the answer by counting backwards. In this question, we’re shown a
number line and it’s got some jumps marked on it, some in orange and some in
red. Can you see all the minus
signs? This tells us that we’re taking
away 10s or ones each time. In other words, we’re counting
backwards. To help us remember this, let’s
draw little arrow heads on the end of each jump. This will show us the direction
that we’re going. So, which number do you think we
started with?
If we’re counting backwards along
our number line in this direction, we must have started with the number 80. So, we know the first number in our
calculation must be 80. And we know that it’s a subtraction
sentence because we’re told so in the question. And also we can see that we’re
counting backwards. Now, where do we end up? What’s the answer to our
subtraction sentence? The answer is the number we
finished on, and we can see this is 27. The subtraction sentence we’re
looking for is going to be 80 take away something which equals 27. But if we look at our three
possible answers, this doesn’t help us. They all begin with 80. They’re all subtractions, and they
all have an answer of 27.
To find the correct answer, we need
to think about the amount that’s being subtracted on the number line. There are one, two, three, four,
five jumps of 10. So, we’ve counted back five 10s and
then one, two, three ones. We know that five 10s are worth 50
and three ones are worth three. If we’ve subtracted 50 and then
three, have we taken away 35, 53, or 63? The answer is 53, isn’t it? The number line shows that we
started with the number 80 and then we subtracted five 10s and three ones, or
53. And we can see that the number we
ended on was 27. The correct subtraction sentence is
80 take away 53 equals 27.
